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Can we measure “wavefunction” of quantum particles?The no-cloning theorem prohibits the copying of an unknown quantum state. Making multiple copies of the same state is required for many experimental systems. If I want a spin-up particle, and I build a system that always generates spin-up particles, then I have made multiple copies of the same state.

Mar16

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Is energy always proportional to frequency?I agree that X is much easier to measure than Y because, for instance, it has a lower mass (X=atom, Y=cat). But you should be careful about saying that something "cannot be measured in principle", which means that it is impossible to elucidate the desired quantity from the experiment on theoretical grounds. Such a statement requires a qualitative, not quantitative, difference. It was long thought that atomic and molecular matter-wave interference would be impossible. I agree that we shouldn't expect a cat-ter wave interferometer any time soon.

Why Does Light Not Become Polarized In A Magnetic and/or Electric Field?@John, if we're discussing the Faraday effect, it's completely incorrect to say that there's no interaction. The whole point is that you can use materials to couple DC magnetic fields and optical fields. If you restrict yourself to a vacuum, you miss most of the interesting phenomena in electromagnetism and I'd be out of a job.

Feb10

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Is a hard drive heavier when it is full?@OBrien, I mean "much greater than". The energy (or energy barrier) needs to be much greater than the thermal energy to prevent temperature-induced changes in the bits. Since we're thinking in terms of an exponential process, we can satisfy $E>>kT$ with $6 >>1$.

Feb5

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Time-reversal symmery and topological insulatorsIt is an interesting topic, and one that makes the "feel" of statistical mechanics very different that other areas of physics. I think many physicists would disagree with the following statement, but I think of the difference between microscopic and macroscopic reversibility as an example of an emergent phenomena.

Why Does Light Not Become Polarized In A Magnetic and/or Electric Field?The Faraday effect in particular is an off-resonant excitation, hence it's only dispersive. Dichroism is a related effect in which the light polarization it changed or rotated through absorption -- this is what I mean by "relaxation". Incidentally, dichroism is an excellent example where the light becomes aligned with the magnetic field: one ciruclar polarization is absorbed and the other one remains. "Information" about the original state is lost. To distinguish these effects, I'm referring to the way that light can be dissipitated, absorbed, or incoherently scattered.